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Hydrogen for sustainable aviation: Where are we now?

Hydrogen is emerging as a game-changing contender for innovation towards meaningful reduction in aircraft emissions. Real-life validation of its potentialities over the next few years will decide the fate of hydrogen in the upcoming era of sustainable aviation. 

ZeroAvia's hydrogen-powered aircraft on one of its test flight. Source: ZeroAvia
To the US-based startup Universal Hydrogen, the 15-minute test flight earlier this year of a DHC Dash 8-300 aircraft powered partly by its retrofitted hydrogen fuel cell powertrain meant the start of a new beginning in the emerging era of zero-emission aviation. Claimed as "the largest fuel-cell powered airplane ever to fly", the aircraft principally flew on the thrust generated by the powertrain (mounted on its right wing) during its maiden trial in March.

California-based Universal Hydrogen is focused on developing hydrogen fuel cell retrofit kits for regional aircrafts like Dash 8s and ATR twin-turboprops, with a hope to penetrate the market in the next 10-15 years. This could potentially act as the first step for developing similar hydrogen powertrain systems for larger (narrow-body) jets such as the Boeing 737 and Airbus A320 families that account for over half the global commercial jet fleet.

Back in January 2023, ZeroAvia, another developer, made news for similar reasons following the first flight of a Dornier 228 test aircraft it had retrofitted with a hydrogen-electric powertrain. ZeroAvia's system, consisting of two fuel cell systems and a lithium-ion battery pack, replaced one of the two engines on the turboprop model. The company's 2-5 MW powertrain program claims to scale the clean engine technology for up to 90-seat aircraft, with further expansion into narrowbody aircraft demonstrators over the next decade. 

Both Universal Hydrogen and ZeroAvia are aiming to obtain certification for their modified aircraft with hydrogen-electric powertrains to fly on commercial routes by 2025. They claim that the hydrogen fuel cell solutions can match the CASM (cost-per seat mile) of jet fuel or SAF (sustainable aviation fuel) to start with, while achieving higher real unit economics over the subsequent years. They even assure that the retrofitted aircraft would require no radical changes to existing airport and fueling infrastructures to handle the new hydrogen hybrid technology.

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A real 'game-changer'

Although hydrogen has always been a 'fancy' fuel to take mankind to the skies since the dawn of the aviation era, it is only in recent years that the fuel has gained momentum as a versatile energy carrier with 'clean' production-use credentials. Given that the aviation industry already represents about 2.5 percent of global greenhouse gas (GHG) emissions and that fleet sizes and operations are consistently increasing, the need for a disruptive, long-term technological innovation is felt widely across the industry and climate action stakeholders.

Hydrogen is emerging as a game-changing contender for innovation towards meaningful reduction in aircraft emissions and a significant reduction (with potential to eliminate) all of aviation's GHG emissions.

With large-scale production of green hydrogen using renewable energy expected over the next decade, hydrogen can become increasingly cost-competitive to fossil-based ATF (air turbine fuel) and bio-mass based SAF. Hydrogen can clean aviation with the application of renewable energy without any major disruption, complementing the existing storage and fueling options at airports, thereby facilitating wide-scale adoption.

Fuel cells and hydrogen combustion (using liquid hydrogen as turbine fuel) are the two technological pathways emerging in this space. The prohibitive weight of battery-electric powertrains and land-use challenges associated with SAF make these hydrogen propulsion options highly plausible, even making zero-emission aviation (using green hydrogen) economically viable in the long-run.

Hydrogen's higher gravimetric energy density also makes it superior to power large aircraft over long distances than jet fuel or lithium-ion batteries. Also, the maturation of these two hydrogen technologies in other industries such as automotive and stationary power will ease up technological innovation and supply chain economics to the benefit of the aviation industry in future. 

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MTU Aero Engines AG is working on various concepts of zero-emission aviation propulsion systems to achieve the aviation industry's big goal of becoming carbon neutral in the coming decades.

Challenges aplenty

Although the most promising zero-emission route for aviation today, hydrogen is facing significant challenges in terms of technological, supply chain and infrastructure.

For instance, storage on board an aircraft is technically difficult, thanks to hydrogen's lower volumetric density, which may even take up to four times larger storage space than jet fuel on an average, thus warranting significant redesigns to traditional airframe and fuel tank designs. Aircraft with hydrogen storage will need to achieve better safety record than conventional airplanes before they can be inducted for passenger transport. 

Further, the cost of hydrogen per kWh is much more expensive than jet fuel at present. We can assume, however, that intensified global generation of renewable energy and mass scaling-up of green hydrogen in the coming years will bring costs down and lower the gap with options such as jet fuel and SAF.

Another big challenge is the logistics of transporting hydrogen and storage infrastructure required at airports to support refueling. Hydrogen can either be transported via existing pipelines in large quantities or through use of trucks to supply smaller quantities. On-site production of hydrogen fuel can be another option at large airports, especially those with abundant renewable energy generation in the locality. But the transition from jet fuel to hydrogen is easier said than done; it requires significant investments and collaborations between companies involved and standardization on the part of regulators for uniformity across the globe.

Despite these constraints, hydrogen is still at the heart of climate-neutral aviation and one can find an emerging consensus among tech suppliers, aircraft manufacturers, airliners, and policymakers on the technological pathways to achieve this revolutionary transition.

For instance, the International Air Transport Association (IATA), a premier association of world's airlines, has recognized liquid hydrogen as a practical alternative fuel for aviation. In the near future, IATA considers fuel cell powertrains suitable for short-range aircraft and hydrogen combustion for long-range and higher-payload operations.

Universal Hydrogen's DHC Dash 8-300 aircraft powered partly by its retrofitted hydrogen fuel cell powertrain. Source: Universal Hydrogen

Ambitions fly high

Aircraft manufacturers have also put forth their ambitions loud and clear. Airbus is taking a giant leap with a vision of developing the world's first zero-emission aircraft by 2035. The company considers hydrogen combustion technology for 'Carbon Zero' scenario (all gross carbon emissions reduced to zero), while finds a hybrid hydrogen setup combining combustion and fuel cell technologies for 'Carbon + Zero', in which NOx and persistent contrails are also significantly reduced along with carbon emissions.

To achieve a 'True Zero' scenario, meaning zero gross emissions, Airbus trusts on the long-term evolution of hydrogen fuel cell and battery-electric technologies. The company has already unveiled 'ZEROe' hydrogen-powered concept aircraft, while a final decision on technology choices and airplane configurations is expected by 2025. Airbus is also working with its airline and airport partners on an initiative called 'Hydrogen Hubs at Airports' to study infrastructure requirements for hydrogen deployment.

Last December, Brazilian aircraft firm Embraer announced the inclusion of hybrid and hydrogen-electric propulsion technologies onto its 'Energia' program meant for future aviation, with 2035 set as the timeline for the roll-out of hydrogen fuel cell aircraft.

Boeing, on the other hand, continues to take a skeptical approach to commercialize hydrogen-powered aircrafts over the next decade, citing "engineering and lifecycle sustainability challenges" that the company claims to actively assess, together with the learnings from its previous five hydrogen demonstration flight test programs. Although the US-based manufacturer has committed itself to achieve net zero carbon emissions by 2050, Boeing believes that the scale-up of SAF offers the "largest potential to decarbonize aviation" over the next 20-30 years.

Nevertheless, it is safe to assume that next year is likely to be a watershed period for the initial validation of the claimed potentialities of hydrogen-based aviation. The road to climate-neutral aviation is long, and perhaps, commercial aircraft powered by hydrogen might not enter passenger service for another decade or so. 

Yet, the real-life validation of hydrogen's potential to make aviation sustainable in the later decades of this century will put the aviation industry onto the right track today towards a universal shift to cleaner solutions.


Rolls-Royce test fires aero engine using Green Hydrogen 

Rolls-Royce and easyJet claim to have set a new aviation milestone with the world's first run of a modern aero engine on hydrogen. The ground test was conducted on an early concept demonstrator using green hydrogen created by wind and tidal power.
Author : Dhiyanesh Ravichandran
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